For a fast exchange of digital signals between frequently changing remote stations, for example between telecommunications terminals, between computers serving for data processing or even between exchange-oriented processors, bus systems are frequently employed in which the respectively connected stations have random access to the bus system via transmission and reception equipment. The digital signals thereby emitted from a station are transmitted on the bus toward both sides and, based on decoding of preceding addresses, are only accepted by the station or stations identified by the address or addresses. What is referred to as CSMA/CD (carrier sense multiple access with collision detection) has gained particular significance for the control of access to the bus system; before a station begins to send, it listens on the bus to see whether a transmission event is already occurring. When there is a transmission, the station initially waits for the end of the transmission, afterwhich the station then begins to send. It initially continues to listen until it is certain, after a defined time span (round trip delay time), that all other stations have received its transmitted signals. When another station has begun to transmit at approximately the same time, then the two stations will identify a collision of their digital signals, whereupon they will respectively abort their transmission operation in order to begin a new transmission after a random time span.
A simple method for identifying multiple transmissions is as follows. The active station compares the signal it is to output to the signal statuses prevailing at its reception equipment which is connected to the bus system, whereby a signal status difference indicates a collision. The recognition of such a signal status difference assumes that one signal status can always physically prevail over the other signal statuses (given binary signals) on the bus system. Regardless of the number of transmitters which are active roughly simultaneously, this signal priority must also be guaranteed for line lengths of more than 100 m for every station.
This prerequisite is not met by present commercially available standard transmitters having two difference outputs for a bus system having two signal conductors. For such bus transmitters which are usually switched into a third, quiescent condition (tri state) in times of inactivity, i.e., outside of the bit time spans of transmitted bits, opposed signal statuses which can lead to signal errors or to transmitter damage due to overload can be avoided in this quiescent condition for clock-controlled or access-controlled bus systems. In a random access bus system, the access control is based on opposing signal statuses, however, such opposing signal statuses may not be recognized under certain conditions when using ordinary standard bus transmitters in a system having long line lengths between stations which are far apart. The opposing signal statuses may be overlooked due to the voltage drop along the bus or line lengths.
Such a standard bus transmitter is not used in a known bus system (see Pct Patent application No. WO-A-84/00862) having two signal conductors in which transmission equipment has two signal statues and has two switches closed or opened synchronously with one another and connected to two signal conductors via two-unequal-voltage sources. One of these two signal statuses effects a dominance condition in the bus system and one of the connected transmission equipment leaves the two signal conductors at least approximately unloaded in the other thereof. Such transmission equipment can be realized with a special transmitter having internal resistance asymmetrical difference outputs; alternatively thereto, it is also possible to realize the switches with two simultaneously conductive or inhibited transistors via which two unequal voltage sources can be connected to the two signal conductors. Such a bus system thus requires bus transmitters having special asymmetries, whereas standard difference transmitters may possibly have internal resistance symmetrical complementary outputs.
Another bus system (disclosed by U.S. Pat. No. 3,671,671) has two signal conductors and does not provide any standard bus transmitters, but provides special transmission equipment each having a constant current source which can be switched on and off by means of a transistor switch.
A further bus system (disclosed in an article in BYTE 6 (1981) Pages 50-60 and, FIG. 1) is known in which a decoupling diode is provided between a sole output of every transmission equipment and a sole signal conductor. This system is non-symmetrical since it comprises only a single signal conductor.
A bus system (disclosed by European response EP-Al-0171555) has two signal conductors with transmission equipment connected thereto via two difference outputs and has two signal statuses one of which effects a dominance signal status in the bus system and for the other of which the transmission equipment leaves the two signal conductors at least approximately unloaded. The two difference outputs at the transmission equipment respectively formed with the bus transmitters are connected to the two signal conductors via two mutually opposite diodes which are in an inhibited condition in the other signal status.
This bus system without excluding use of tri-state transmitters, and which also enables a utilization of simple bus transmitters (without tri-state) having symmetrically executed complementary outputs, guarantees a uniform signal status in the bus system independent of the extent of the bus system and of the distance between the plurality of transmitters which are roughly simultaneously active. Also an overload of the transmitter outputs for opposing signal statuses, is avoided. An increased reliability in the recognition of a collision between transmitters given simultaneous access to the bus is guaranteed. The system, however, presumes a galvanic coupling of the transmission-reception equipment to the two signal conductors.
Such a galvanic coupling of the transmission-reception equipment to the signal conductors of a bus system, however, is frequently undesired and the object of the present invention is to specify transmission-reception equipment for a bus system having two signal conductors which enables a fast collision recognition given access of the bus system even without such galvanic coupling, with simple means and with high reliability.